Conventionally, there have been various proposals for speed change control of vehicle stepless speed change mechanisms. For example, types in which the engine rotation speed is set to a target value in accordance with the speed of the vehicle and control to change the speed change ratio is carried out to make the actual engine rotation speed equal to the target value, and types in which the speed change ratio of a stepless speed change device is set to a target value in accordance with the speed of the vehicle in a similar manner and control to change the speed change ratio is carried out to make the speed change ratio equal the target value are known. In order to carry out speed change control or other controls on a stepless speed change device in accordance with the operating conditions of the vehicle, a plurality of sensors is provided to measure the rotation speed of the rotating members that constitute the power transmission path from the engine to the wheels.
In a speed change control device, the vehicle speed is obtained from the rotation speed of the rotating members that constitute the power transmission path. The vehicle speed obtained is used as an input parameter for setting the target value in speed change control. Generally the vehicle speed is obtained from the rotation speed of the rotating member on the output side of a clutch mechanism used for controlling the starting of the vehicle and the like. In other words, the vehicle speed is obtained from the rotation speed immediately prior to being transmitted to the wheels. Conventionally, a control device is known in which back up control (fail safe control) is carried out by detecting the breakdown of the sensor that outputs the measurement value for obtaining the vehicle speed. When a breakdown is detected, the vehicle speed is obtained alternatively from the measurement value of a sensor that detects the rotation speed of another rotating member. Then the speed change control continues to be carried out using this vehicle speed (see for example, Japanese Patent Application Laid-open No. S63-74735).
In the conventional form of speed change control, during the time after a breakdown occurs and until the breakdown is detected, the back up control is started, and its effect starts to operate, speed change control is carried out that is not suitable for the operating conditions based on the measurement value of the sensor with a breakdown. In this way, the fuel consumption and running properties could have become worse due to the vehicle running with a speed change ratio that is not appropriate to the operating conditions. In particular, because detection of breakdown of the sensor that measures the rotation speed of the wheels is not carried out using a rotating member of the speed change mechanism, this time period tends to be long.